Metrology generally involves measuring various physical features of a target component. For example, structural and material characteristics (e.g. material composition, dimensional characteristics of structures and/or critical dimensions of structures, etc.) of the target component can be measured using metrology systems. In the example of semiconductor metrology, various physical features of a fabricated semiconductor component may be measured using a metrology system.
Currently, metrology systems employ either libraries or regression for performing the measurement. However, each of these measurement processes inherently exhibit their own respective limitations, such that there is a need for addressing these and/or other issues associated with the prior art.
For example, when making a measurement, non-linear least square is often used to fit a set of observed signals collected from one or a combined set of metrology tools with calculated signals from a non-linear model containing a set of floating parameters, including those characteristics to be measured. If Maxwell's equations are solved vigorously in real-time to obtain the calculated signals for a corresponding floating parameter set, it is referred to measurement with real-time regression (or RTR). On the other hand, a parametric non-linear model may be created offline to approximate the solutions of Maxwell's equations. During the measurement, the parametric model can be used for fast and accurate approximation to the calculated signals. The parametric model is called a library, and its corresponding measurement is denoted as a library measurement.
RTR measurement generally uses a large computing farm to evaluate theoretical signals for given set of parameters by vigorously solving Maxwell's equations in real-time. RTR measurement can quickly adapt to process changes without a need of lengthy library changes. Therefore, the time to make process changes and to keep measurement is short. Unfortunately, however, it requires a more expensive computing platform to support real-time computations. Furthermore, overall throughput of the metrology system is slowed down by the calculations linked to solving Maxwell's equations in real-time. This results in a high cost of metrology using RTR, both from the perspective of computations and a number of metrology tools required.
Library measurement is generally very fast and only needs a workstation or a server to provide adequate computing power. However, in the event of large process changes that are beyond the pre-defined range of parameters in the library, it will take a significant amount of time to change and re-qualify a new library in order to recover the measurement credibility. Until the measurement creditability is recovered, the measurement process is at risk of an excursion that cannot be detected since no metrology is available (i.e. the measurement is generally skipped), possibly impacting the yield on a significant number of components under measurement.